An image sensor is a semiconductor device for converting an optical image into an electrical signal. The image sensor is roughly classified as a charge coupled device (CCD) image sensor or a complementary metal oxide semiconductor (CMOS) image sensor (CIS).
The CIS includes photodiodes and MOS transistors in a unit pixel, and sequentially detects electrical signals of respective unit pixels in a switching manner to realize an image. Particularly, the unit pixel of a typical CIS includes a photodiode region receiving a light signal to convert the light signal into an electrical signal and a transistor region processing this electrical signal.
In a horizontal CMOS image sensor, the photodiode and the transistor are horizontally formed adjacent to each other on a substrate. Accordingly, an additional region for a photodiode is required on the substrate for each unit pixel, which reduces a fill factor and limits possibility of resolution.